Theodore Hardeen (March 4, 1876 – June 12, 1945) was a magician and escape artist, best known as Harry Houdini's brother. So dedicated was he to Houdini that Hardeen usually introduced himself as the “brother of Houdini." He was the founder of the Magician's Guild.

He was born as Ferencz Dezso Weisz or Weisz Ferenc Dezső in Budapest, Hungary and went by the name of Theodore Weiss when the family was living in Appleton, Wisconsin. He was known as Deshi and later Dash by his parents. It was Hardeen who first conceived of escaping from a straitjacket in full view of the audience -- a concept fully used by Houdini and generations of performers since.

Hardeen learnt his business from Houdini, and he learnt it well, as no man could have failed to do with such a master. But the natural ability was always there. Even if Houdini had not existed to point the way, it is quite likely that Hardeen would have become an escape artiste. I have seen him perform several times, and have marvelled at his achievements. The huge chains in the cartoon are hardly an exaggeration of those from which he breaks loose. The handcuffs but a simple thing compared with those with which he is used to toy. Hardeen, like his more famous brother Harry Houdini is a great favourite in the magical world. He is a man of considerable culture and one of the best raconteurs I have ever met. Like Houdini, he is willing to expend both time and money freely to further the cause of magic. Both the amateurs and the professionals of our number regard him as a friend. And the general public regard him as a brilliant artiste and a thrilling entertainer.

Hardeen passed many items to his associate Douglas Geoffrey Mackintosh who was an illusionist and performed under the name Douglas Geoffrey and after Hardeen passed away, as "Hardeen, Jr".[1][2]